On blackening steel with the hot oil, Use boiled linseed oil, or any cocking oil. The problem with using used motor oil now a days whether it is car or truck is the heavy metals in it, and other contaminants that can be converted into fumes. As most will tend to over heat the metal to orange to red hot which isn't needed.
You shouldn't use synthetic oils as the polymers and other stuff in them can cause headaches and respiratory discomfort.
The biggest difference in the food grade oils, is the smoke point. With canola oil being one of the highest.
You only have to heat the steel up to around 500'/600'F and then spray, dip or brush(with a natural bristle brush, not plastic). You can repeat as needed to get the shade you need.
Also the various food type oils will give slightly different colors and shades from brown to black.
And of course for any one making BarBQue equipment or other food contact pieces. The use of a food grade oil is a must, it is also recommended for anything that a child will come in contact with as most things make it in to their mouths. Sooner than later.